1. Field of the Invention
The present invention relates to a display device for displaying an image by using a display panel of the matrix type, with picture elements disposed in an array of matrix intersections, such as a matrix-type liquid-crystal panel or a matrix-type electroluminescent display panel; more particularly, it relates to a matrix-type display device for use in mobile information-terminal equipment, such as a mobile telephone set, that displays moving images.
2. Description of Related Art
Display devices employing matrix-type liquid crystals and the like have hitherto been used in portable information-processing equipment such as mobile telephone sets and mobile information-terminal equipment.
A basic requirement of recent mobile telephones, for example, is a battery-driven operating time of several hundred hours in the state in which a so-called standby screen is displayed. In the matrix-type display devices used in mobile telephones, therefore, a frame memory, separate from the graphics memory that has the role of input buffering of image data, is often built into the circuit for driving the liquid-crystal display panel, to reduce power consumption by making image data transfer unnecessary when a still image is displayed. That is, when a still image is displayed, these devices do not consume power by transferring data to the circuit for driving the liquid-crystal display panel; large numbers of lower-power liquid-crystal matrix-type display devices configured in this way have been used in mobile telephones in recent years.
Low-cost STN (super-twisted birefringent) liquid-crystal panels with built-in frame memories as described above, which are still lower in power consumption, have frequently been used as liquid-crystal display panels for mobile telephones. However, a videophone function is expected to be added in the future, together with the start of moving-picture distribution service conforming to the IMT-2000 standard. A moving-image display will then be necessary, and since the conventional STN liquid-crystal panel has inadequate response speed, a changeover to display panels that support moving-image displays is foreseen for mobile telephones. Specifically, it is foreseen that active-matrix liquid-crystal panels such as TFT (Thin Film Transistor) liquid-crystal panels and MIM (Metal Insulator Metal) liquid-crystal panels, which have a high response speed and good image quality, will be primarily used.
The active-matrix liquid-crystal panels that are expected to be used in the future are not, in general, as low in power consumption as the STN liquid-crystal panels that have been used in the past. Active-matrix liquid-crystal panels with power consumption reduced to a level permitting use in mobile telephones have been developed in recent years, however.
As for STN liquid-crystal panels, although their future use has become uncertain because of their comparatively slow response speed, fast-response STN liquid-crystal panels with response speeds increased to enable the display of moving images are being developed.
Organic electroluminescent (EL) panels, which employ a display method in which the picture-element section itself is made to emit light, have a much faster response speed than liquid-crystal panels, and since these displays panels are of the self-luminous type, they do not require illumination such as back-lighting or front-lighting, so their power consumption is not very high. Accordingly, organic EL display panels are considered suitable as display panels for mobile telephones because they can be slimmed and lightened by the amount taken up by back-lighting or other illumination.
The general response speeds of the display panels described above are about 300-500 msec for the STN liquid-crystal panels that have been used in mobile telephones, about 30-50 msec for an active-matrix liquid-crystal panel such as a TFT, about 70-80 msec for a fast-response STN liquid-crystal panel, and on the order of several microseconds for an organic EL panel.
FIG. 9 is a block diagram showing the structure of a conventional matrix-type display device with a built-in frame memory.
In the matrix-type display device 9 in FIG. 9, reference numeral 70 denotes an input control section that controls the timing etc. of input image data, and reference numeral 80 denotes a display-panel module that displays the input image data.
The input control section 70 has a graphics memory 11 that can temporarily store input image data at least in frame units, a data-write control circuit 12 comprising a microprocessor or the like with an address bus, a data bus, control signal lines, and the like, that carries out control when the input image data are written in the graphics memory 11, and a data-read control circuit 13 that reads the image data temporarily stored in the graphics memory 11 and transfers the data to the display-panel module 80.
The display-panel module 80 has: a frame memory 21 that can store image data transferred from the input control section 70 in at least frame units; a display panel 22 in which picture-element units are provided at intersections in a matrix formed by a plurality of signal lines laid out in parallel columns and a plurality of signal lines laid out in parallel rows; a signal-electrode driving circuit 23 that generates a clock signal as a reference for displaying an image on the display panel 22 and, based on the clock signal, generates control signals for reading image data from the frame memory 21 and driving the signal lines of the display panel 22, and generates a frame synchronization signal and a line synchronization signal of the display panel 22; and a scan-electrode driving circuit 24 that generates control signals based on the frame synchronization signal and line synchronization signal to drive the scanning lines of the display panel 22. The display panel 22 is, for example, a liquid-crystal display panel with liquid-crystal display elements disposed in a matrix array.
The image data input to the matrix-type display device 9 from the outside and written in the graphics memory 11 are GD1; the image data read from the graphics memory 11 and transferred to the frame memory 21 are GD2; the image data read from the frame memory 21 and input to the signal-electrode driving circuit 23 are GD3. The frame synchronization signal output from the signal-electrode driving circuit 23 to the scan-electrode driving circuit 24 is FS; the line synchronization similarly output from the signal-electrode driving circuit 23 to the scan-electrode driving circuit 24 is LS; the read control signal likewise output from the signal-electrode driving circuit 23 to read the stored contents of the frame memory 21 is RC.
The operation of the matrix-type display device 9 will be described with reference to the image-data transfer timing diagram in FIG. 10, as well as to FIG. 9.
Image data GD1 are input to the input control section 70 of the matrix-type display device 9 from the outside by a communication function or the like and stored temporarily in the graphics memory 11 under control of the data-write control circuit 12. When the process of storing the image data GD1 in the graphics memory 11 ends at timing t1, those image data are immediately read out by the data-read control circuit 13 and transferred to the frame memory 21 as image data GD2, as shown in FIG. 10.
In the display-panel module 80, the image data stored in the frame memory 21 are read out periodically by the signal-electrode driving circuit 23 as image data GD3, in a refresh cycle based on an independently generated clock signal, as shown in FIG. 10, and are input to the signal-electrode driving circuit 23. Using the independent clock, the signal-electrode driving circuit 23 generates the read control signal RC and sends it to the frame memory 21, generates and outputs control signals for the signal electrodes of the matrix display panel 22, and generates a frame synchronization signal FS and line synchronization signal LS and sends them to the scan-electrode driving circuit 24. The scan-electrode driving circuit 24 generates and outputs control signals for the scanning electrodes of the matrix display panel 22, based on the frame synchronization signal FS and line synchronization signal LS.
FIGS. 11A to 11C are drawings showing a thick vertical line moving from the left edge toward the right edge on the matrix display panel 22 of the matrix-type display device 9.
The frame frequency of the display panel 22 is generally about sixty frames per second, several times the frequency of data transfer from the graphics memory 11 to the frame memory 21. The transfer of image data GD2 is carried out asynchronously with respect to the readout of image data GD3 from the frame memory 21 to the matrix display panel 22. If the image data GD3 read from the frame memory 21 for each frame are, in proceeding temporal order, the n-th frame, the (n+1)-th frame, and the (n+2)-th frame, as shown in FIG. 10, then the image of the n-th frame, with the vertical line 100a, is first displayed continuously as in FIG. 11A.
Next, at timing t2 in the (n+1)-th frame in FIG. 10, since image data GD2 and GD3 are not synchronized, the writing of image data GD2 overtakes and passes the readout of image data signal GD3. Thus as shown in FIG. 11B, below timing t2 in the vertical scanning direction, the image of vertical line 100b in the (n+1)-th frame becomes the image of the newly written vertical line 101a, creating a discontinuous offset in the vertical line. This offset disappears in the (n+2)-th frame shown in FIG. 11C, in which there is only the newly written vertical line 10b. 
Thus in the conventional matrix-type display device 9 shown in FIG. 9, because the image data GD2 are transferred from the graphics memory 11 to the frame memory 21 asynchronously with respect to the frame cycle of the matrix display panel 22, a situation arises in which the image frame displayed on the display panel 22 switches midway through to the image of the next frame.
This type of situation also occurs when the conventional STN liquid-crystal panel, which has a slow response speed, is used as the matrix display panel 22. Compared with the one-frame image data transfer time in the display panel, however, the response time of the liquid crystal in the conventional STN liquid-crystal panel is so long that a problem occurs: even if image data are transferred frame by frame to display a full-motion moving image, the liquid crystal cannot respond in time to produce an adequate display, so even if a vertical offset occurred in an image because the next image was transferred midway through the display of one frame on the liquid-crystal display panel, display of the image had usually already become impossible, so the problem was comparatively unnoticeable and was ignored.
Nevertheless, when a display panel with a comparatively fast response speed, such as an active-matrix liquid-crystal panel, a fast-response STN panel, an organic EL panel or the like is used in a mobile telephone set in order to display moving images as described above, for example, for images with image data moving in the horizontal direction as shown in FIG. 11B, since the problem of response speed has been eliminated, the problem of one frame changing midway through to the next frame and a vertical offset occurring in the image becomes apparent. As a result, the quality of the displayed moving image is markedly degraded. Accordingly, when a display panel with a comparatively fast response speed is used in a mobile telephone or the like, the problem of the occurrence of vertical offsets in the image becomes a problem that cannot be ignored.
An object of the present invention is to improve the display of moving images in mobile information-terminal equipment.
The invented matrix-type display device has a matrix display panel and a frame memory. A signal-electrode driving circuit generates a frame synchronization signal and a line synchronization signal, and generates control signals for reading the image data from the frame memory and driving the signal lines of the matrix display panel. From the frame synchronization signal and line synchronization signal, a scan-electrode driving circuit generates control signals that drive the scanning electrodes of the matrix display panel. Frames of image data read from the frame memory are thereby displayed on the matrix display panel.
The invented matrix-type display device also has a graphics memory for temporary buffering of input image data, a data-write control circuit that controls the writing of image data into the graphics memory, and a data-read control circuit that transfers the image data from the graphics memory to the frame memory. The data-write control circuit outputs a write-end signal at the completion of the writing of a frame of image data into the graphics memory.
The invented matrix-type display device further includes a synchronizing circuit that generates a read-start signal from the first frame synchronization signal occurring after a write-end signal. The read-start signal causes the read-control circuit to start transferring image data from the graphics memory to the frame memory.
The writing of image data into the frame memory is thereby synchronized with the reading of image data out of the frame memory. The synchronization is arranged so that the write address never overtakes the read address during the reading of a frame of image data. When a moving image is displayed, accordingly, each individual frame is displayed correctly, with no mixing of data from two consecutive frames.
The invented matrix-type display device may also have a delay circuit that delays the frame synchronization signal before input to the synchronizing circuit. The delay can be set to provide optimal read-write synchronization for the frame memory, for various different types of matrix display panels.
The delay circuit preferably also receives the line synchronization signal, and delays the frame synchronization signal by a predetermined number of line synchronization pulses. Optimal read-write synchronization of the frame memory can then be maintained despite clock-signal frequency variations.
The matrix display panel may be, for example, a liquid-crystal display panel of the reflective type, the reflective semi-transmissive type, the active-matrix type, or the fast-response super-twisted birefringent type. Alternatively, the matrix display panel may be an organic electroluminescent panel or an active-matrix organic electroluminescent panel.